Engine exhaust systems utilize sensors to detect operating conditions and adjust engine air-fuel ratio. One type of sensor used is a switching type heated exhaust gas oxygen sensor (HEGO). The HEGO sensor provides a high gain between measured oxygen concentration and voltage output. The HEGO can provide an accurate indication of the stoichiometric point, but provides air/fuel information over an extremely limited range (other than indicating lean or rich).
Another type of sensor used is a universal exhaust gas oxygen sensor (UEGO). The UEGO sensor can operate across a wide range of air-fuel ratios, for example from 10:1 (rich) to pure air (lean). However, as a result, the voltage to oxygen concentration has a lower gain. Furthermore, the UEGO sensor may not provide an indication of stoichiometry as precise as the HEGO sensor, especially under widely varying temperature conditions.
The inventors herein have recognized that when an oxygen sensor is used in a post catalyst position, the precise indication of stoichiometry given by the HEGO sensor provides advantageous results, but the limited bandwidth degrades the capability of the control system to provide fast convergence to desired operating conditions. Likewise, using an UEGO sensor can provide advantageous information when operating away from stoichiometry, however, catalyst efficiency when operating about stoichiometry can degrade due to the imprecise measurement of the stoichiometric point.
One approach to try and correct for the UEGO sensor inaccuracies near stoichiometry is described in U.S. Publication 2001/0052473. Here, the power supply to the pump current is cut off, and a correction value is then determined. However, the inventors herein have also recognized a disadvantage with such an approach. For example, the power supply can be turned off only in limited conditions, such as deceleration fuel shut-off, and thus an accurate reading of stoichiometry is only available under select conditions. Furthermore, the select conditions typically do not include operation at stoichiometry under feedback control. As such, the measurement comes at an inappropriate time and is not available when needed most. Further, errors due to variations in temperature can change depending on engine conditions, and as such even if this correction is used, errors persist.
To overcome these disadvantages, and harness the respective advantages of the above sensors, the following approach can be utilized. Specifically, in one aspect, a sensor is used that comprises: a first reference cell having a reference voltage; a second pumping cell having a pumping current, and a circuit configured to pump current in the pumping cell in a first direction to prevent the reference voltage from increasing higher than a first voltage limit; and to pump current in the pumping cell in a second direction to prevent the reference voltage from decreasing lower than a second voltage limit. In one example, when the circuit pumps current in the pumping cell in the first direction to prevent the reference voltage from increasing higher than the first voltage limit, the circuit allows the reference voltage to decrease lower than the first voltage limit. Likewise, when the circuit pumps current in the pumping cell in the second direction to prevent the reference voltage from decreasing lower than the second voltage limit, the circuit allows the reference voltage to increase higher than the second voltage limit.
In this way, the reference voltage can be driven by chemical reactions to equilibrate and provide an accurate indication of stoichiometry, similar to a HEGO sensor. Likewise, outside of stoichiometry, the reference voltage is controlled in a one-sided fashion via positive and negative pumping current at respective voltage limits to provide an indication of air-fuel ratio over a wide range.
An advantage of such operation is the ability to provide a signal that is both accurate at stoichiometry and indicative of air-fuel ratio over a wider range. Such operation leads to more accurate feedback air-fuel ratio control at stoichiometry with high gain sensing, while still providing air-fuel feedback information outside of stoichiometry, such as for lean burn operation.
In another aspect, a method is provided for sensing an air-fuel ratio of exhaust gasses from an engine using a sensor having a pumping cell and a reference cell. The method comprises:
pumping current in the pumping cell during at least a first set of operating conditions;
reducing said pumping during at least a second set of operating conditions;
providing a signal from said sensor during at least said first and second operating conditions; and
adjusting at least one of a fuel injection amount and an air amount into the engine to maintain a desired air-fuel ratio based on said signal during at least said first and second operating conditions.
As such, the method advantageously uses a sensor that both (1) pumps current in the pumping cell during at least a first set of operating conditions (such as to provide an indication of air-fuel ratio over a wide range), and (2) reduces said pumping during at least a second set of operating conditions (such as about stoichiometry to allow chemical equilibrium to drive a reference voltage). In this way, by using a signal from the sensor in both circumstances to provide feedback air-fuel ratio control, accurate control can be obtained both about stoichiometry, and away from stoichiometry. Increased catalyst efficiency and reduced emissions can also be obtained.